Share prices are determined by the dealers in order to make sure there are as many people wanting to buy as there are people wanting to sell. You can only sell shares if there are people willing to buy them, and you can only buy shares if there are people willing to sell them.
If more people are wanting to buy, then the price increases (to encourage others to be prepared to sell them). If more people are wanting to sell, then the price falls (to encourage others to be prepared to buy them).
Also…..a big disposal of shares suggests that the seller thinks there are going to be problems with the company in the future. This in itself is going to depress the share price.